Pressure sensing apparatus and method

ABSTRACT

An apparatus (22) for processing signals from a touch panel (10) is described. The touch panel (10) includes a layer of piezoelectric material (16) disposed between a plurality of sensing electrodes (14, 20) and at least one common electrode (15). The apparatus (22) includes a first circuit (23) for connection to the plurality of sensing electrodes (14, 20). The first circuit (23) is configured to generate one or more first pressure signals (29). Each first pressure signal (29) corresponds to one or more sensing electrodes (14, 20) and is indicative of a pressure acting on the touch panel (10) proximate to the corresponding one or more sensing electrodes (14, 20). The apparatus also includes a second circuit (24) for connection to the at least one common electrode (15). The second circuit (24) is configured to generate a second pressure signal (30) indicative of a total pressure applied to the touch panel (10). The apparatus also includes a controller (25) configured to determine an estimate of the total pressure based on a weighted difference of the second pressure signal (30) and a sum over the one or more first pressure signals (29).

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for processingsignals from a pressure-sensing touch panel, and to touch panel systemsusing the apparatus and method.

BACKGROUND

Resistive and capacitive touch panels are used as input devices forcomputers and mobile devices. One type of capacitive touch panel,projected capacitance touch panels, is often used for mobile devices. Anexample of a projected capacitance touch panel is described in US2010/0079384 A1.

Projected capacitance touch panels operate by detecting changes inelectric fields caused by the proximity of a conductive object. Thelocation at which a projected capacitance touch panel is touched isoften determined using an array or grid of capacitive sensors. Althoughprojected capacitance touch panels can usually differentiate betweensingle-touch events and multi-touch events, they suffer the drawback ofnot being able to sense pressure. Thus, projected capacitance touchpanels tend to be unable to distinguish between a relatively light tapand a relatively heavy press. A touch panel which can sense pressure canallow a user to interact with a device in new ways by providingadditional information about user interaction(s) with the touch panel.

WO 2016/102975 A2 describes apparatus and methods for combinedcapacitance and pressure sensing in which a single signal is amplifiedthen subsequently separated into pressure and capacitance components. WO2017/109455 A1 describes apparatus and methods for combined capacitanceand pressure sensing in which a single signal is separated into acapacitance signal, and a pressure signal which is amplified.

SUMMARY

According to a first aspect of the invention, there is providedapparatus for processing signals from a touch panel. The touch panelincludes a layer of piezoelectric material disposed between a pluralityof sensing electrodes and at least one common electrode. The apparatusincludes a first circuit for connection to the plurality of sensingelectrodes. The first circuit is configured to generate one or morefirst pressure signals. Each first pressure signal corresponds to one ormore sensing electrodes and is indicative of a pressure acting on thetouch panel proximate to the corresponding one or more sensingelectrodes. The apparatus also includes a second circuit for connectionto the at least one common electrode. The second circuit is configuredto generate a second pressure signal indicative of a total pressureapplied to the touch panel. The apparatus also includes a controllerconfigured to determine an estimate of the total pressure based on aweighted difference of the second pressure signal and a sum over the oneor more first pressure signals.

Each sensing electrode may contribute to a single first pressure signal.

The controller may be further configured to determine a location atwhich pressure is applied to the touch panel. A coefficient used for theweighted difference of the second pressure signal and the sum over theone or more first pressure signals may depend upon the location.

The controller may determine the estimate of the total pressure usingthe equation:

F _(CE)=(1−C _(CE))Q _(CE) −C _(CE) Q _(sen)

in which F_(CE) is a piezoelectric charge induced on the at least onecommon electrode, Q_(CE) is a charge measured on the at least one commonelectrode, Q_(sen) is the sum of charges measured on all of theplurality of sensing electrodes, and C_(CE) is a pre-calibrated constanthaving a value between zero and unity. The estimate of the totalpressure may be based on F_(CE).

The controller may be further configured to determine, for each of atleast one first pressure signal, an estimate of the pressure acting onthe touch panel proximate to the corresponding one or more sensingelectrodes, based on the first pressure signal, the second pressuresignal and the total pressure.

The controller may be configured to determine a location at whichpressure is applied to the touch panel. One or more coefficients used todetermine the estimate of the pressure acting on the touch panelproximate to the one or more sensing electrodes may depend upon thelocation.

The touch panel may includes a number N of sensing electrodes, and thecontroller may determine the estimate of the pressure acting on thetouch panel proximate to the one or more sensing electrodes using theequation:

$F_{n} = {Q_{n} - {\frac{k_{n}}{C_{CE}}( {Q_{CE} - F_{CE}} )}}$

in which F_(n) is a piezoelectric charge induced on the n^(th) of Nsensing electrodes, F_(CE) is a piezoelectric charge induced on the atleast one common electrode, Q_(n) is a charge measured on the n^(th) ofN sensing electrodes, Q_(CE) is a charge measured on the at least onecommon electrode, C_(CE) is a pre-calibrated constant having a valuebetween zero and unity, and k_(n) is a pre-calibrated constantcorresponding to the n^(th) of N sensing electrodes and having a valuebetween zero and unity. The estimate of the pressure acting on the touchpanel proximate to the one or more sensing electrodes may be based onone or more corresponding values of F_(n).

The first circuit may also be configured to generate, for each firstelectrode, a capacitance signal indicative of a capacitance of thesensing electrode. The controller may be configured to determine alocation at which pressure is applied to the touch panel based on thecapacitance signals.

Generating the first pressure signals and the capacitance signals mayinclude separating single signals received from the sensing electrodes.

Each first pressure signal may correspond to a single sensing electrode.

The first circuit may include a capacitive touch controller forconnection to the sensing electrodes. The first circuit may include acharge amplifier for connection to each of the sensing electrodes via animpedance network. The charge amplifier may be configured to output afirst pressure signal corresponding to all of the sensing electrodes.The second circuit may include a common electrode charge amplifier forconnection to the at least one common electrode. The common electrodecharge amplifier may be configured to generate the second pressuresignal.

The apparatus may further include a differential amplifier configured toreceive the first pressure signal and the second pressure signal, and tooutput a weighted difference of the first pressure signal and the secondpressure signal to the controller. The controller may be configured todetermine the estimate of the total pressure based on the weighteddifference received from the differential amplifier.

A touch panel system may include the apparatus, and a touch panelincluding a layer of piezoelectric material disposed between a pluralityof sensing electrodes and at least one common electrode.

An electronic device may include the touch panel system.

According to a second aspect of the invention, there is provided amethod of processing signals from a touch panel. The touch panelincludes a layer of piezoelectric material disposed between a pluralityof sensing electrodes and at least one common electrode. The methodincludes generating one or more first pressure signals. Each firstpressure signal is based on signals received from one or more sensingelectrodes. Each first pressure signal is indicative of a pressureacting on the touch panel proximate to the corresponding one or moresensing electrodes. The method also includes generating, based onsignals received from the at least one common electrode, a secondpressure signal indicative of a total pressure applied to the touchpanel. The method also includes determining an estimate of the totalpressure based on a weighted difference of the second pressure signaland a sum over the one or more first pressure signals.

The method may also include determining a location at which pressure isapplied to the touch panel. A coefficient used for the weighteddifference of the second pressure signal and the sum over the one ormore first pressure signals may depend upon the location.

Determining the estimate of the total pressure applied to the touchpanel may include using the equation:

F _(CE)=(1−C _(CE))Q _(CE) −C _(CE) Q _(sen)

in which F_(CE) is a piezoelectric charge induced on the at least onecommon electrode, Q_(CE) is a charge measured on the at least one commonelectrode, Q_(sen) is the sum of charges measured on all of theplurality of sensing electrodes and C_(CE) is a pre-calibrated constanthaving a value between zero and unity. The estimate of the totalpressure may be based on F_(CE).

The method may also include determining, for each of at least one firstpressure signal, an estimate of the pressure acting on the touch panelproximate to the corresponding one or more sensing electrodes, based onthe first pressure signal, the second pressure signal and the totalpressure.

The method may also include determining a location at which pressure isapplied to the touch panel. One or more coefficients used to determinethe estimate of the pressure acting on the touch panel proximate to theone or more sensing electrodes may depend upon the location.

The touch panel may include a number N of sensing electrodes.Determining the estimate of the pressure acting on the touch panelproximate to the one or more sensing electrodes may include using theequation:

$F_{n} = {Q_{n} - {\frac{k_{n}}{C_{CE}}( {Q_{CE} - F_{CE}} )}}$

in which F_(n) is a piezoelectric charge induced on the n^(th) of Nsensing electrodes, F_(CE) is a piezoelectric charge induced on the atleast one common electrode, Q_(n) is a charge measured on the n^(th) ofN sensing electrodes, Q_(CE) is a charge measured on the at least onecommon electrode, C_(CE) is a pre-calibrated constant having a valuebetween zero and unity, and k_(n) is a pre-calibrated constantcorresponding to the n^(th), of N sensing electrodes and having a valuebetween zero and unity. The estimate of the pressure acting on the touchpanel proximate to the one or more sensing electrodes may be based onone or more corresponding values of F_(n).

The method may also include generating, based on signals received fromeach sensing electrode, a capacitance signal indicative of a capacitanceof the sensing electrode. The method may also include determining alocation at which pressure is applied to the touch panel based on thecapacitance signals.

Generating the first pressure signals and the capacitance signals mayinclude separating single signals received from the sensing electrodes.

Each first pressure signal may correspond to a single sensing electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention will now be described, byway of example, with reference to the accompanying drawings in which:

FIG. 1 is an equivalent circuit diagram of a piezoelectric sensor;

FIG. 2 is a circuit diagram of a first measurement circuit;

FIG. 3 is a circuit diagram of a second measurement circuit;

FIG. 4 is a circuit diagram of a third measurement circuit;

FIG. 5 is a circuit diagram of a fourth measurement circuit;

FIG. 6 is a circuit diagram of a fifth measurement circuit;

FIG. 7 is a cross-sectional view of a first touch panel forpiezoelectric pressure measurements;

FIG. 8 illustrates a first apparatus for differential piezoelectricpressure measurements;

FIGS. 9 and 10 illustrate methods of obtaining differentialpiezoelectric pressure measurements;

FIG. 11 shows measured signals corresponding to externally inducedcharges on a sensing electrode and a common electrode;

FIG. 12 shows measured signals corresponding to piezoelectric inducedcharges on a sensing electrode and a common electrode;

FIG. 13 illustrates a second apparatus for differential piezoelectricpressure measurements;

FIGS. 14A to 14C illustrate separating superposed capacitive andpiezoelectric pressure signals using an analog-to-digital convertorsynchronised to a driving signal;

FIG. 15 illustrates an exemplary configuration of charge amplifiers fordifferential piezoelectric pressure measurements;

FIG. 16 is a plan view of a second touch panel for piezoelectricpressure measurements;

FIG. 17 is a plan view of a third touch panel for piezoelectric pressuremeasurements;

FIG. 18 illustrates a third apparatus for differential piezoelectricpressure measurements;

FIG. 19 illustrates a fourth apparatus for differential piezoelectricpressure measurements; and

FIG. 20 illustrates a fifth apparatus for differential piezoelectricpressure measurements.

DETAILED DESCRIPTION

In the following description, like parts are denoted by like referencenumerals.

In some circumstances, a variety of unwanted signals may couple via auser's digit or conductive stylus to the sensing electrodes of apiezoelectric pressure sensing touch panel or a combined capacitance andpiezoelectric pressure sensing touch panel. Such signals may beamplified along with the desired piezoelectric pressure signals, and maybe of comparable or larger amplitude than piezoelectric pressuresignals. For example a user's digit placed on a piezoelectric pressuresensing touch panel or a combined capacitance and piezoelectric pressuresensing touch panel sensor may couple mains interference into thesensing electrodes. Additionally or alternatively, a user may becomecharged with static electricity, which may couple to the sensingelectrodes of a piezoelectric pressure sensing touch panel or a combinedcapacitance and piezoelectric pressure sensing touch panel.

Piezoelectric sensors are two electrode devices with a typically highoutput impedance at low frequencies, which may cause piezoelectricsensors to become vulnerable to picking up interference from externalelectric fields. The desired signals generated on the two electrodes ofa piezoelectric sensor due to mechanical strain are of oppositepolarity. By contrast, interference due to coupling to external electricfields will be of the same polarity on both electrodes. The presentspecification describes methods and apparatus for combining the signalsfrom electrodes arranged on opposite sides of a layer of piezoelectricmaterial, so that interference from coupling to external electric fieldsmay be reduced or removed, whilst retaining or enhancing the desiredpiezoelectric pressure signals.

Piezoelectric Sensing

Referring to FIG. 1, an equivalent circuit of a piezoelectric sensor 1is shown.

A piezoelectric sensor 1 may be modelled as a voltage source, V_(piezo),in series with a capacitor, C_(piezo). The capacitance C_(piezo)represents the capacitance between first and second electrodes which arearranged with piezoelectric material between them. The voltage sourceV_(piezo) represents an open circuit voltage generated across thecapacitance C_(piezo) when a force is applied to the piezoelectricsensor 1.

Referring also to FIG. 2, a first example of a measurement circuit 2 isshown.

The first measurement circuit 2 includes a single-ended amplifier A1having inputs connected across the piezoelectric sensor 1 and a feedbacknetwork in the form of a resistor R_(fb) and capacitor C_(fb) connectedin parallel across the output and the inverting input of the amplifierA1. In practice, the first measurement circuit 2 may include furtherpassive components, switches for resetting the feedback network, and soforth. Depending upon the specific configuration used, the firstmeasurement circuit 2 may measure a voltage, a current, a charge or acombination thereof.

Referring also to FIG. 3, a second example of a measurement circuit 3 isshown.

The second measurement circuit 3 is the same as the first measurementcircuit 2, except that the non-inverting input of the single-endedamplifier A1 is grounded, rather than connected to an electrode of thepiezoelectric sensor 1. In this way, the second measurement circuit 3measures the current I₁ flowing into the inverting input which is atground potential. This configuration of the second measurement circuitmay reduce or eliminate the effect of parasitic capacitances. In idealcircumstances, the measured current I₁ is substantially equal to aninduced piezoelectric current signal I_(piezo), i.e. I₁≈I_(piezo).Typically, the second measurement circuit 3 is configured to integratethe current signal I₁ in order to provide on the output V_(out1) acharge signal corresponding to a charge Q_(piezo) induced across thepiezoelectric sensor 1. In other words, V_(out1) is functionally relatedto the piezoelectric charge Q_(piezo), which in turn is functionallyrelated to a force applied to the piezoelectric sensor 1.

Referring also to FIG. 4, a third example of a measurement circuit 4 isshown.

The third measurement circuit 4 is the same as the second measurementcircuit 3, except that it includes an equivalent circuit 5 representingcapacitive coupling to an external source of electromagneticinterference, V_(int).

A potential issue with single-ended amplifiers A1 is that externalelectric fields may induce a charge on the amplifier input that may beinterpreted as a piezoelectric pressure signal. This problem may occurin the piezoelectric force sensors of touch screens for piezoelectricpressure sensing or touch screens for combined capacitive touch andpiezoelectric pressure sensing. A users' digit or conductive stylusapplying the force to be measured is typically separated from theelectrodes forming a piezoelectric force sensor by one or several thinlayers of glass and/or plastic. A users' digit or conductive stylus maybe at a different potential to the electrodes forming a piezoelectricforce sensor. Such potential differences may arise due to, for example,electrostatic charging or coupling to other electrical sources, forexample, pick-up induced by a mains power supply.

In the third measurement circuit 4, an interfering electromagneticsource V_(int) couples to both electrodes of the piezoelectric sensor 1via a pair of capacitances C_(int1) and C_(int2). Consequently, themeasured signal I₁ is a superposition of the desired piezoelectricpressure signal I_(piezo) and an unwanted interference signal I_(int1),i.e. I₁=I_(piezo)+I_(int1). The inclusion of the interference signalcomponent I_(int1) in the measured signal I₁ may cause errors indetermining an applied force, for example false detection of an appliedforce and/or causing the smallest reliably measureable increment ofapplied force to be increased.

Differential Measurements

The current flow induced in response to polarisation P of piezoelectricmaterial between a pair of first and second electrodes forming apiezoelectric sensor 1 has the opposite sense in each of the first andsecond electrodes. By contrast, interference signals induced by externalsources V_(int) will have the same sign for the first and secondelectrodes forming a piezoelectric sensor 1.

Referring also to FIG. 5, a fourth example of a measurement circuit 6 isshown.

In the fourth measurement circuit 6, a first single-ended amplifier A1has one input connected to a first electrode 7 of the piezoelectricsensor 1 to receive a first measurement current I₁, and the other inputof the first amplifier A1 is grounded. Similarly, a second single-endedamplifier A2 has one input connected to a second electrode 8 of thepiezoelectric sensor 1 to receive a second measurement current I₂, andthe other input of the second amplifier A2 is grounded. A thirdsingle-ended amplifier A3 has one input connected to the output V_(out1)of the first amplifier A1 and the other input connected to the outputV_(out2) of the second amplifier A2. Each of the amplifiers A1, A2, A3has a respective resistive-capacitive feedback network R_(fb1)-C_(fb1),R_(fb2)-C_(fb2), R_(fb3)-C_(fb3).

The interfering source V_(int) is capacitively coupled to the firstelectrode 7 by a first capacitance C_(int1) and to the second electrode8 by a second capacitance C_(int2). As discussed hereinbefore, thecurrent flow I_(piezo) induced in response to polarisation P ofpiezoelectric material between the first and second electrode 7, 8 hasthe opposite sense in each of terminals 7, 8, whereas interferencesignals L_(int1), I_(int2) induced by the interfering source V_(int)will have the same sign. Accordingly, the first and second measurementcurrents may be approximated as:

I ₁ =I _(int1) +I _(piezo)

I ₂ =I _(int2) −I _(piezo)   (1)

The third amplifier A3 is used to obtain a difference, and whenI_(int1)≈I_(int2), the output V_(out) of the third amplifier A3 will berelated to:

I ₁ −I ₂≈2I _(piezo)   (2)

In this way, by measuring the current flowing from both electrodes 7, 8of the piezoelectric sensor 1 it is possible to determine a measure ofthe piezoelectric current I_(piezo) in which the influence of theinterfering source V_(int) is reduced or removed.

In the general case, if C_(int1)≠C_(int2) and I_(int1)≠I_(int2), aweighted difference may be used. For example, if I_(int1)=α·I_(int2), inwhich α is a scalar constant determined from calibration experiments,then the influence of an interfering source V_(int) may be reduced orremoved by obtaining:

I ₁ −αI ₂=(1+α)I _(piezo)   (3)

In general, obtaining the difference of the measured signals I₁, I₂ maybe performed by specifically configured circuits at the analogue signallevel, or by post-processing following conversion to digital signals.

It is not necessary to know the absolute values of the interferencecapacitors C_(int1), C_(int2). From Equation (3), it may be observedthat all that is needed is the ratio α of noise introduced on the firstelectrode 7 to that introduced to the second electrode 8. The ratio αmay be obtained from calibration experiments, for example, bydeliberately introducing a test signal that mimics an interferencesignal V_(int) to the system and recording the response of the first andsecond measurement currents I₁, I₂ in the absence of applying any forceto the piezoelectric sensor 1, i.e. such that I₁=I_(int1) andI₂=T_(int2). This information may be used to determine the correctionratio as α=I₁/I₂.

In practice, the correction may be performed by obtaining a differenceof the first and second amplifier A1, A2 outputs V_(out1), V_(out2) inthe fourth measurement circuit 6. This may be calibrated in the same wayby obtaining a ratio of the outputs V_(out1), V_(out2) in response to atest signal and in the absence of any force applied to the piezoelectricsensor 1. If the ratio β=V_(out1)/V_(out2) determined from calibrationis not approximately unity, then a weighted differenceV_(out1)−β·V_(out2) may be obtained by inserting appropriate impedancesbetween the outputs V_(out1), V_(out2) of the first and secondamplifiers A1, A2 and the respective inputs of the third amplifier A3.Alternatively, the third amplifier A3 may be omitted and a weighteddifference V_(out1)−β·V_(out2) may be obtained by processing in thedigital signal domain.

Referring also to FIG. 6, a fifth example of a measurement circuit 9 isshown.

In the fifth measurement circuit 9, a differential amplifier DA1 has oneinput connected to the first electrode 7 and the other input connectedto the second electrode 8. The reduction or removal of influence of aninterfering source V_(int) may be implemented in the analogue domain bysetting the values of a first feedback network R_(fb1), C_(fb1) and asecond feedback network R_(fb2), C_(fb2) according to the ratio betweenthe interfering capacitances C_(int1), C_(int2). For example, byselecting C_(fb1)/C_(fb2)=C_(int1)/C_(int2). Such selection may beperformed through calibration experiments similar to those describedhereinbefore and by using, for example, trimmer capacitors to providethe feedback capacitances C_(fb1), C_(fb2).

Differential Measurements in Touch Panels for Piezoelectric PressureMeasurements

In the examples described hereinbefore, differential measurements havebeen described in relation to piezoelectric sensors 1 in which the firstand second electrodes 7, 8 may be substantially co-extensive and ofsimple geometry. Such a configuration permits relatively simpledifferential measurements. However, in a practical touch panel forpiezoelectric pressure measurements or combined capacitance andpiezoelectric pressure measurements, a first electrode 7 may be one ofmany electrodes which share a common second electrode 8. Additionally,in some example a first electrode 7 may be an electrode whichadditionally functions as a receiving, Rx, and/or transmitting, Tx,electrode of a capacitance measurement system. In such touch panels, thesecond electrode 8 may be a common counter electrode having a relativelylarger, or much larger, total area than each of a number of firstelectrodes 7. Providing separate, matched counter electrodes for each Rxand/or Tx electrode would require an additional patterned conductivelayer or layers, and the associated electrical connections.Consequently, simple differential measurements as illustrated inrelation to the fourth or fifth measurement circuits 6, 9 may not bepractical.

Instead, the present specification describes methods for obtainingdifferential measurements of piezoelectric signals from a touchscreenfor piezoelectric pressure measurements or for combined capacitance andpiezoelectric pressure measurements which includes at least one,unpatterned common electrode (corresponding to the second electrode 8,sometimes referred to as a counter-electrode). The methods of thepresent specification are also applicable (with minor modifications) totouchscreens for piezoelectric pressure measurements or for combinedcapacitance and piezoelectric pressure measurements in which there aretwo or more second electrodes 8, each being common to two or more firstelectrodes 7.

First Apparatus

Referring to FIG. 7, a first example of a touch panel 10 forpiezoelectric pressure measurements or combined capacitive andpiezoelectric pressure measurements is shown.

The first touch panel 10 includes a first layer structure 11 having afirst face 12 and a second, opposite, face 13. A number of first sensingelectrodes 14 are disposed on the first face 12 of the first layerstructure 11. Each of the first sensing electrodes 14 extends (orequivalently is elongated) in a first direction x, and the first sensingelectrodes 14 are spaced apart in a second direction y. A commonelectrode 15 is disposed to substantially cover the second face 13 ofthe first layer structure 11.

The first layer structure 11 includes one or more layers, including atleast a layer of piezoelectric material 16. Each layer included in thefirst layer structure 11 is generally planar and extends in first x andsecond y directions which are perpendicular to a thickness direction z.The one or more layers of the first layer structure 11 are arrangedbetween the first and second faces 12, 13 such that the thicknessdirection z of each layer of the first layer structure 11 issubstantially perpendicular to the first and second faces 12, 13.

The first touch panel 10 also includes a second layer structure 17having a first face 18 and a second, opposite, face 19. A number ofsecond sensing electrodes 20 are disposed on the first face 18 of thesecond layer structure 17. Each of the second sensing electrodes 20extends (or equivalently is elongated) in the second direction y, andthe second sensing electrodes 20 are spaced apart in a first directionx.

The second layer structure 17 includes one or more dielectric layers 21.Each dielectric layer 21 is generally planar and extends in first x andsecond y directions which are perpendicular to a thickness direction z.The one or more dielectric layers 21 of the second layer structure 17are arranged between the first and second faces 18, 19 of the secondlayer structure 17 such that the thickness direction z of eachdielectric layer 21 of the second layer structure 17 is perpendicular tothe first and second faces 18, 19.

Preferably, the layer of piezoelectric material 16 includes or is formedof a piezoelectric polymer such as polyvinylidene fluoride (PVDF) orpolylactic acid. However, the layer of piezoelectric material 16 mayalternatively be a layer of a piezoelectric ceramic such as leadzirconate titanate (PZT). Preferably, the first and second sensingelectrodes 14, 20, and the common electrode 15 are formed from silvernanowires. However, the first and second sensing electrodes 14, 20, andthe common electrode 15 may alternatively be formed of transparentconductive oxides such as indium tin oxide (ITO) or indium zinc oxide(IZO). The first and second sensing electrodes 14, 20, and the commonelectrode 15 may be metal films such as aluminium, copper, silver orother metals suitable for deposition and patterning as a thin film. Thefirst and second sensing electrodes 14, 20, and the common electrode 15may be conductive polymers such as polyaniline, polythiphene,polypyrrole or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT/PSS). The first and second sensing electrodes 14, 20, and thecommon electrode 15 may be formed from a metal mesh, metallic nanowires,graphene, and/or carbon nanotubes. The dielectric layer(s) 21 mayinclude layers of a polymer dielectric material such as polyethyleneterephthalate (PET) or layers of pressure sensitive adhesive (PSA)materials. However, the dielectric layer(s) 21 may include layers of aceramic insulating material such as aluminium oxide.

The first layer structure 11 may include only the layer of piezoelectricmaterial 16 such that the first and second opposite faces 12, 13 arefaces of the piezoelectric material layer 16. Alternatively, the firstlayer structure 11 may include one or more dielectric layers 21 whichare stacked between the layer of piezoelectric material 16 and the firstface 12 of the first layer structure 11. The first layer structure 11may include one or more dielectric layers 21 stacked between the secondface 13 of the first layer structure 11 and the layer of piezoelectricmaterial 16.

The second layer structure 17 may include only a single dielectric layer21, such that the first and second faces 18, 19 of the second layerstructure 17 are faces of a single dielectric layer 21.

Alternatively, a second layer structure 17 need not be used (see FIG.17), and the second sensing electrodes 20 may be disposed on the firstface 12 along with the first sensing electrodes (FIG. 17).

In FIG. 7, the first touch panel 10 has been shown with reference toorthogonal axes labelled x, y, and z. However, the first, second andthickness directions need not form a right handed orthogonal set.

Referring also to FIG. 8, a first apparatus 22 for differentialpiezoelectric pressure measurements or combined capacitance anddifferential piezoelectric pressure measurements is shown.

The first apparatus 22 includes the first touch panel 10, a firstcircuit 23, a second circuit 24 and a controller 25. Each of the firstand second sensing electrodes 14, 20 is connected to the first circuit23 by a corresponding conductive trace 26. The common electrode 15 isconnected to the second circuit 24.

The first circuit 23 receives from, and may optionally transmit signalsto, the first and second sensing electrodes 14, 20. The first circuit 23measures a number of first piezoelectric pressure signals 29. The firstcircuit 23 is connectable to each of the first and second sensingelectrodes 14, 20, in groups or individually. Each first piezoelectricpressure signal 29 corresponds to one or more of the first or secondsensing electrodes 14, 20, and each first piezoelectric pressure signal29 is indicative of a pressure acting on the touch panel 10 proximate tothe respective one or more first or second sensing electrodes 14, 20.For example, the first circuit may measure or generate a firstpiezoelectric pressure signal 29 corresponding to each first sensingelectrode 14 and a first piezoelectric pressure signal 29 correspondingto each second sensing electrode 20. Alternatively, each firstpiezoelectric pressure signal 29 may correspond to a pair of adjacentfirst or second sensing electrodes 14, 20, and so forth. Each sensingelectrode 14, 20 contributes to one first piezoelectric pressure signal29.

Optionally, the first circuit 23 may also measure mutual capacitancesignals 27 corresponding to each intersection 28 of the first and secondsensing electrodes 14, 20. In other examples, the first circuit 23 mayinstead measure self-capacitance signals corresponding to each first andsecond sensing electrode 14, 20. The first circuit 23 may determine thecapacitance signals 27 and the first piezoelectric pressure signals 29concurrently. Alternatively, the first circuit 23 may alternate betweendetermining the capacitance signals 27 and the first piezoelectricpressure signals 29.

For example, the first circuit 23 may be configured for combinedcapacitance and piezoelectric pressure measurements as described in WO2016/102975 A2, the entire contents of which are incorporated herein byreference. In particular, the first circuit 23 may be configured asdescribed in relation to examples shown FIGS. 21 to 26 of WO 2016/102975A2. Alternatively, the first circuit 23 may be configured for combinedcapacitance and piezoelectric pressure measurements as described in WO2017/109455 A1, the entire contents of which are incorporated herein byreference. In particular, the first circuit 23 may be configured asdescribed in relation to examples shown in FIGS. 4 to 21 of WO2017/109455 A1. In other examples, the first circuit 23 may beconfigured as described hereinafter with reference in particular toFIGS. 13 to 17.

However, the methods of the present specification are not limited tothese examples, and are applicable to any first circuit 23 which iscapable of providing the hereinbefore described functions.

The second circuit 24 measures a second piezoelectric pressure signal 30which corresponds to the common electrode 15. The second piezoelectricsignal 30 should be indicative of a total pressure applied to the touchpanel 10. When more than one common electrode 15 is used, a secondpiezoelectric signal 30 may be generated corresponding to each commonelectrode 15, for subsequent summation by the controller 25.Alternatively, when more than one common electrode 15 is used, thesecond circuit 24 may generate a single second piezoelectric signal 30based on charges induced on all the common electrodes 15. Under idealconditions and in the absence of external interference, a sum over thesecond piezoelectric pressure signals 30 and the first piezoelectricsignals 29 should be approximately zero (up to a measurement error)because the sensing electrodes 14, 20 and the common electrode(s) 15 arearranged on opposite sides of any polarisation P induced in the layer ofpiezoelectric material 16.

The piezoelectric pressure signals 29, 30, and optionally thecapacitance signals 27, are produced in response to a user interactionwith the first touch panel 10, or with a layer of material overlying thefirst touch panel 10. In the following description, reference to a “userinteraction” shall be taken to include a user touching or pressing atouch panel 10 or an overlying layer of material. The term “userinteraction” shall be taken to include interactions involving a user'sdigit or a stylus (whether conductive or not). The term “userinteraction” shall also be taken to include a user's digit or conductivestylus being proximate to a touch sensor or touch panel without directphysical contact (i.e. zero or negligible applied pressure).

The controller 25 receives the first and second piezoelectric pressuresignals 29, 30, and generates corrected piezoelectric pressure values 32which are output to a processor (not shown) which operates a deviceincorporating the first apparatus 22. The controller 25 generates acorrected piezoelectric pressure value 32 in the form of an estimate ofthe total pressure applied to the touch panel 10, based on a weighteddifference of the second pressure signal 30 and a sum over the firstpressure signals 29. For example, the controller 25 may generate acorrected piezoelectric pressure value 32 using Equations (13) or (21).

The controller 25 may also generate an estimate of the pressure actingon the touch panel proximate to a first sensing electrode 14, a group offirst sensing electrodes 14, or each first sensing electrode 14, basedon the respective first pressure signal(s) 29, the second pressuresignal 30 and the total pressure. For example, the controller 25 maygenerate one or more corrected piezoelectric pressure values 32 usingEquations (24) or (26) described hereinafter. Additionally oralternatively, the controller 25 may also generate an estimate of thepressure acting on the touch panel proximate to a second sensingelectrode 20, a group of second sensing electrodes 20, or each secondsensing electrode 20, based on the respective first pressure signal(s)29, the second pressure signal 30 and the total pressure. For example,the controller 25 may generate corrected one or more piezoelectricpressure values 32 using Equations (23) or (25) described hereinafter.

The controller 25 may additionally or alternatively relay the raw firstand/or second piezoelectric pressure signals 29, 30 to a processor (notshown) which operates a device (not shown) incorporating the firstapparatus 22. In some examples, the processor (not shown) mayalternatively perform some or all of the described functions of thecontroller 25.

In some examples, the controller 25 may determine touch location data 31based on the first and second pressure signals 29, 30. The touchlocation data 31 indicates the locations, for example x, y coordinates,of one or more user interactions. The touch location data 31 is outputto the processor (not shown) which operates the device (not shown)incorporating the first apparatus 22. Coefficients used for generatingcorrected piezoelectric pressure values 32 in the form of an estimate ofthe total pressure, a pressure acting on the touch panel proximate toone or more first sensing electrodes 14, and/or a pressure acting on thetouch panel proximate to one or more second sensing electrodes 20, maydepend upon the location x, y.

When measured, the controller 25 receives the capacitance signals 27 andeither relays them to a processor (not shown) which operates a device(not shown) incorporating the first apparatus 22, or performs furtherprocessing of the capacitance values 27. For example, the controller 25may process the capacitance values 27 to generate the touch locationdata 31 for output to the processor (not shown) which operates thedevice (not shown) incorporating the first apparatus 22. Capacitancesignals 27 may permit more accurate determination of the touch locationdata 31 than the first and second pressure signals 29, 30 alone.

First Method of Measurement

Referring also to FIGS. 9 and 10, a first method of performingdifferential piezoelectric pressure measurements will be described.

An object 33, for example a user's digit, which is proximate to ortouching the touch panel 10 may become charged to a potential V_(int) byelectrostatic charging or from acting as an antenna for a source V_(int)of electromagnetic interference. There is a capacitive coupling C_(ext)between the object 33 and the overall assemblage of all of the sensingelectrodes 14, 20 and the common electrode 15. A total electrostaticcharge Q_(ES) is induced in the overall assemblage of all of the sensingelectrodes 14, 20 and the common electrode 20 as approximatelyQ_(ES)=C_(ext)·V_(int). It should be noted that it may not be possibleto calibrate C_(ext) in practice, because the precise geometry will becontinually changing as a user moves their digit and/or stylus inrelation to the touch panel 10, and also will vary between differentusers and different digits of the same user. Additionally, V_(int) maynot be measureable in general.

The first method of the specification is based on the premise that anunknown, total electrostatic charge Q_(ES) induced on the electrodes 14,15, 20 will be made up of a sum of individual electrostatic chargesinduced on each of the electrodes 14, 15, 20.

Hereinafter, the m^(th) of M first sensing electrodes 14 mayalternatively be denoted as y_(m) and the n^(th) of N second sensingelectrodes 20 may alternatively be denoted as x_(n). If theelectrostatic charge induced on the n^(th) of N second sensingelectrodes 20, x_(n) by the object 33 is denoted Sx_(n) and so forth,the electrostatic charge induced on the m^(th) of M first sensingelectrodes 14, y_(m) by the object 33 is denoted Sy_(m) and so forth,and the electrostatic charge induced on the counter electrode 15 by theobject 33 is denoted S_(CE), then the total electrostatic charge Q_(ES)may be approximated as:

$\begin{matrix}{Q_{ES} = {{\sum\limits_{n = 1}^{N}{Sx_{n}}} + {\sum\limits_{m = 1}^{M}{Sy_{m}}} + S_{CE}}} & (4)\end{matrix}$

The electrostatic charges Sx_(n), Sy_(m), S_(CE) induced on individualelectrodes x_(n), y_(m), 15 may alternately be expressed as fractions ofthe total electrostatic charge induced Q_(ES). For example, theelectrostatic charge Sx_(n) may be written as Sx_(n)=k_(n)·Q_(ES), inwhich k_(n) is the fraction of the total electrostatic charge Q_(ES)induced on the n^(th) of N second sensing electrodes x_(n). Similarly,the electrostatic charge Sy_(m) may be written as Sy_(m)=h_(m)·Q_(ES),in which h_(M) is the fraction of the total electrostatic charge Q_(ES)induced on the m^(th) of M first sensing electrodes y_(m). Further, theelectrostatic charge S_(CE) may be written as S_(CE)=C_(CE)·Q_(ES), inwhich C_(CE) is the fraction of the total electrostatic charge Q_(ES)induced on the counter electrode 15. Substituting these expressions intoEquation (4):

$\begin{matrix}{{Q_{ES} = {{\sum\limits_{n = 1}^{N}{k_{n}Q_{ES}}} + {\sum\limits_{m = 1}^{M}{h_{m}Q_{ES}}} + {C_{CE}Q_{ES}}}}{1 = {{\sum\limits_{n = 1}^{N}k_{n}} + {\sum\limits_{m = 1}^{M}h_{m}} + C_{CE}}}{1 = {C_{x} + C_{y} + C_{CE}}}} & (5)\end{matrix}$

in which:

$\begin{matrix}{{C_{x} = {\sum\limits_{n = 1}^{N}k_{n}}},{C_{y} = {\sum\limits_{m = 1}^{M}h_{m}}}} & (6)\end{matrix}$

In general, the fractions k_(n), h_(m), C_(x), C_(y) and C_(CE) will allbe a function of the touch coordinates, denoted x, y, at which theobject 33 contacts the touch panel 10. In other words, the fractionk_(n) is typically not a constant, and may be a function k_(n)(x,y) ofthe touch coordinates x, y. Similarly, the other fractions may also befunctions of touch position, namely h_(m)(x,y), C_(x)(x,y), C_(y)(x,y)and C_(CE)(x,y). The fractions k_(n), h_(m), C_(x), C_(y) and C_(CE) maybe calibrated by performing appropriate calibration experiments withknown V_(int) and known touch positions x, y. However, because of theposition dependent nature of the fractions k_(n), h_(m), C_(x), C_(y)and C_(CE), full calibration may require a large number of calibrationexperiments to be performed for a given touch panel 10.

When polarisation P of the piezoelectric material layer 16 is inducedbetween the common electrode 15 and the sensing electrodes x_(n), y_(m),the charges induced on the sensing electrodes x_(n), y_(m) have oppositepolarity to the charges induced on the common electrode 15. In otherwords, external coupling to the object 33 induces charge flow betweensystem ground or common mode voltage and the overall assemblage of allof the electrodes x_(n), y_(m), 15, whereas by contrast a polarisation Pof the piezoelectric material layer 16 induces charge to flow betweenthe counter electrode 15 and the sensing electrodes x_(n), y_(m). Oneconsequence, as explained hereinbefore, is that charges induced by thepolarisation P of the piezoelectric material layer 16 are expected tosum to zero, at least to within a measurement error.

If the piezoelectric charge induced on the n^(th) of N second sensingelectrodes x_(n) by a polarisation P of the piezoelectric material layer16 is denoted Fx_(n) and so forth, the piezoelectric charge induced onthe m^(th) of M first sensing electrodes y_(n) by a polarisation P ofthe piezoelectric material layer 16 is denoted Fy_(m) and so forth, andthe piezoelectric charge induced on the counter electrode 15 by apolarisation P of the piezoelectric material layer 16 is denoted F_(CE),then a total induced piezoelectric charge Q_(PT) may be approximated as:

$\begin{matrix}{Q_{PT} = {0 = {{\sum\limits_{n = 1}^{N}{Fx}_{n}} + {\sum\limits_{m = 1}^{M}{Fy}_{m}} + F_{CE}}}} & (7)\end{matrix}$

It may be noted that the piezoelectric charge F_(CE) induced on thecounter electrode 15 may provide a good measure of the total forceapplied to the touch panel 10.

Referring in particular to FIG. 10, the charge induced on the n^(th) ofN second sensing electrodes x_(n), 20 may be written as:

Qx _(n) =Sx _(n) +Fx _(n)

Qx _(n) =k _(n) Q _(ES) +Fx _(n)   (8)

Similarly, the charge induced on the m^(th) of M first sensingelectrodes y_(m), 14 may be written as:

Qy _(m) =h _(m) Q _(ES) +Fy _(m)   (9)

and the charge induced on the counter electrode 15 may be written as:

Q _(CE) =C _(CE) Q _(ES) +F _(CE)   (10)

In the first method of measurement, the charges Qx_(n), Qy_(m) measuredby all of the sensing electrodes x_(n), y_(m) are summed to yield:

$\begin{matrix}{{{Q_{sen} = {{\sum\limits_{n = 1}^{N}{Qx_{n}}} + {\sum\limits_{m = 1}^{M}{Qy_{m}}}}}{Q_{sen} = {{\sum\limits_{n = 1}^{N}( {{k_{n}Q_{ES}} + {Fx_{n}}} )} + {\sum\limits_{m = 1}^{M}( {{h_{m}Q_{ES}} + {Fy_{m}}} )}}}}{Q_{sen} = {{Q_{ES}( {{\sum\limits_{n = 1}^{N}k_{n}} + {\sum\limits_{m = 1}^{M}h_{m}}} )} + {\sum\limits_{n = 1}^{N}{Fx_{n}}} + {\sum\limits_{m = 1}^{M}{Fy_{m}}}}}{Q_{sen} = {{Q_{ES}( {1 - C_{CE}} )} - F_{CE}}}} & (11)\end{matrix}$

In which Q_(sen) is the sum of all the charges measured by all of thesensing electrodes x_(n), y_(m), and in which Equations (5) and (7) havebeen employed to obtain the final expression for the Q_(sen). The totalinduced electrostatic charge Q_(ES) may be eliminated between Equations(10) and (11) to yield:

$\begin{matrix}{\frac{Q_{CE} - F_{CE}}{C_{CE}} = {Q_{ES} = \frac{Q_{sen} + F_{CE}}{( {1 - C_{CE}} )}}} & (12)\end{matrix}$

Which may re-arranged for the piezoelectric charge F_(CE) induced on thecounter electrode 15 as:

F _(CE)=(1−C _(CE))Q _(CE) −C _(CE) Q _(sen)   (13)

In which the charge Q_(CE) induced on the counter electrode 15 may bemeasured, the summed charge Q_(sen) on the sensing electrodes x_(n),y_(n) may be obtained by summing all of the measured charges Qx₁, Qx₂, .. . , Qx_(N) and Qy₁, Qy₂, . . . , Qy_(M). The fraction C_(CE) may bedetermined in advance through calibration experiments using an object 33charged, connected or coupled to a known interfering potential V_(int)and at a known location x, y with respect to the touch panel 10. Asmentioned hereinbefore, the fraction C_(CE) is in general a function oftouch location x, y, i.e. C_(CE)=C_(CE)(x,y). The appropriate value ofC_(CE)(x,y) may be obtained, for example, by using capacitance signals27 or touch location data 31 to provide the touch location x, y.Alternatively, in a piezoelectric only touch panel system, the touchlocation x, y may be inferred from the raw first piezoelectric signals29.

In this way, using the first method and Equation (13), a totalpiezoelectric charge F_(CE) may be determined in which the effects ofexternal electrical interference from an object 33 may be reduced oreliminated. The total piezoelectric charge F_(CE) depends on thestraining of the piezoelectric material layer 16, and hence depends onthe force applied to the touch panel 10 by a user input.

In practice, the charges Qx_(n), Qy_(m), Q_(CE) may be detected usingcharge amplifiers 34, such that a voltage output corresponding to then^(th) of N second sensing electrodes x_(n), 20 is Vx_(n) and is relatedto Qx_(n) and so forth. Commonly, a charge amplifier 34 will integratethe input current. For example, if the current on the n^(th) of N secondsensing electrodes x_(n) is Ix_(n), then the voltage Vx_(n) on then^(th) of N second sensing electrodes x_(n) at a time t may, under idealconditions, be expressed as:

$\begin{matrix}{{Vx_{n}} = {{Gx_{n}Qx_{n}} = {Gx_{n}{\int\limits_{0}^{t}{{{Ix}_{n}(\tau)}d\tau}}}}} & (14)\end{matrix}$

In which Gx_(n) is the gain of the n^(th) of N charge amplifiers 34connected to the N second sensing electrodes x_(n) and τ is anintegration variable. Similarly, the voltage on the m^(th) of M firstsensing electrodes y_(m) may be expressed as:

$\begin{matrix}{{Vy_{m}} = {{Gy_{m}Qy_{m}} = {Gy_{m}{\int\limits_{0}^{t}{{{Iy}_{m}(\tau)}d\;\tau}}}}} & (15)\end{matrix}$

In which Gy_(m) is the gain of the m^(th) of M charge amplifiers 34connected to the M first sensing electrodes y_(n), Iy_(m) is the currenton the m^(th) of M first sensing electrodes y_(m) and τ is anintegration variable. Similarly, the voltage on the common electrode maybe expressed as:

$\begin{matrix}{V_{CE} = {{G_{CE}Q_{CE}} = {G_{CE}{\int\limits_{0}^{t}{{I_{CE}(\tau)}d\tau}}}}} & (16)\end{matrix}$

In which G_(CE) is the gain of the charge amplifier 34 connected to thecommon electrode 15, I_(CE) is the current on the common electrode 15and τ is an integration variable. The charge amplifier signals 34corresponding to all of the sensing electrodes x_(n), y_(m) may then beslimmed to yield a summed voltage signal, V_(sen):

$\begin{matrix}{{V_{sen} = {{\sum\limits_{n = 1}^{N}{Vx}_{n}} + {\sum\limits_{m = 1}^{M}{Vy}_{m}}}}{V_{sen} = {{\sum\limits_{n = 1}^{N}{{Gx}_{n}{Qx}_{n}}} + {\sum\limits_{m = 1}^{M}{{Gy}_{m}{Qy}_{m}}}}}} & (17)\end{matrix}$

If the gains are all substantially equal such that Gx_(n)≈Gy_(m)≈G, withG denoting a common gain value, then Equation (17) may be simplified to:

$\begin{matrix}{{V_{sen} = {G( {{\sum\limits_{n = 1}^{N}{Qx_{n}}} + {\sum\limits_{m = 1}^{M}{Qy_{m}}}} )}}{V_{sen} = {GQ_{sen}}}{V_{sen} = {G( {{Q_{ES}( {1 - C_{CE}} )} - F_{CE}} )}}} & (18)\end{matrix}$

Similarly, if G_(CE)≈G, then Equation (10) may be re-written in terms ofthe corresponding charge amplifier 34 output as:

V_(CE)=GQ_(CE)

V _(CE) =G(C _(CE) Q _(ES) +F _(CE))   (19)

Eliminating Q_(ES) between Equations (18) and (19), the voltage analogueof Equation (12) may be obtained as:

$\begin{matrix}{\frac{\frac{V_{CE}}{G} - F_{CE}}{C_{CE}} = {Q_{ES} = \frac{\frac{V_{sen}}{G} + F_{CE}}{( {1 - C_{CE}} )}}} & (20)\end{matrix}$

Equation (20) may equally be obtained by simply substitutingQ_(CE)=V_(CE)/G and Q_(sen)=V_(sen)/G into Equation (12). Re-arrangingEquation (20) for F_(CE), or equivalently substituting Q_(CE)=V_(CE)/Gand Q_(sen)=V_(sen)/G into Equation (13), yields an expression for thepiezoelectric charge F_(CE) induced on the counter electrode 15 in termsof charge amplifier 34 voltage outputs as:

$\begin{matrix}{F_{CE} = {{( {1 - C_{CE}} )\frac{V_{CE}}{G}} - {C_{CE}\frac{V_{sen}}{G}}}} & (21)\end{matrix}$

Thus, it is apparent that provided the charge amplifier 34 gains areapproximately equal to a common gain, G, i.e. Gx_(n)≈Gy_(m)≈G_(CE)≈G,then the relationships derived in terms of induced charges may beequally applicable to the corresponding outputs of charge amplifiers 34.Of course, perfect identity would not be expected in practicalcircumstances because the charge amplifier 34 gains Gx_(n), Gy_(m),G_(CE) will not be perfectly identical to a common gain value G.Furthermore, each charge amplifier 34 will in practice experience DCoffsets and drift, in addition to time-dependent decay of low frequencyand DC components in the voltage output (sometimes referred to as“roll-off”). Nonetheless, provided that the charge amplifier 34 gainsare approximately equal to a common gain value G, i.e.Gx_(n)≈Gy_(m)≈G_(CE)≈G, Equation (21) may be used to generate acorrected signal 32 in which the influence of coupling to externalelectrical fields may be at least partially cancelled.

Second Method of Measurement

The second method of measurement is an extension of the first method ofmeasurement, and may be used to estimate values of piezoelectric chargeFx_(n), Fy_(m) for individual sensing electrodes x_(n), y_(m), based ona weighted correction using the charge Q_(CE) measured on the counterelectrode 15 and the estimated piezoelectric charge F_(CE) on thecounter electrode 15.

Referring again to Equation (8), for the n^(th) of N second sensingelectrodes x_(n), 20:

Qx _(n) =k _(n) Q _(ES) +Fx _(n)   (8)

Referring again to Equation (10), for the counter electrode 15:

Q _(CE) =C _(CE) Q _(ES) +F _(CE)   (10)

Eliminating the total electrostatic charge Q_(ES) between Equations (8)and (10):

$\begin{matrix}{\frac{{Qx_{n}} - {Fx_{n}}}{k_{n}} = {Q_{ES} = \frac{Q_{CE} - F_{CE}}{C_{CE}}}} & (22)\end{matrix}$

Which may be re-arranged for the piezoelectric charge Fx_(n) of then^(th) of N second sensing electrodes x_(n), 20 as:

$\begin{matrix}{{Fx_{n}} = {{Qx_{n}} - {\frac{k_{n}}{C_{CE}}( {Q_{CE} - F_{CE}} )}}} & (23)\end{matrix}$

In which the charge Q_(CE) induced on the counter electrode 15 may bemeasured and the charge Qx_(n) induced on the n^(th) of N second sensingelectrodes x_(n), 20 may be measured. The total piezoelectric chargeF_(CE) may be determined from the first method using Equation (13). Thefractions k_(n) and C_(CE) may be determined in advance throughcalibration experiments performed using an object 33 charged, connectedor coupled to a known interfering potential V_(int), whilst the object33 is arranged at a known location x, y with respect to the touch panel10. As mentioned hereinbefore, the fractions k_(n) and C_(CE) are ingeneral functions of touch location x, y, i.e. k_(n)=k_(n)(x,y) andC_(CE)=C_(CE)(x,y). The appropriate values of k_(n)(x,y) and C_(CE)(x,y)may be obtained by using the capacitance signals 27 or touch locationdata 31 to provide the touch location x, y.

In this way, a piezoelectric charge Fx_(n) for the n^(th) of N secondsensing electrodes x_(n), 20 may be estimated for which the effects ofexternal interference from a non-ground potential of the object 33 arereduced or eliminated.

Similarly, for the m^(th) of M first sensing electrodes y_(m), 14, apiezoelectric charge Fy_(m) for which the effects of externalinterference from a non-ground potential of the object 33 are reduced oreliminated may be obtained using:

$\begin{matrix}{{Fy_{m}} = {{Qy}_{m} - {\frac{h_{m}}{C_{CE}}( {Q_{CE} - F_{CE}} )}}} & (24)\end{matrix}$

In practice, the charges Qx_(n), Qy_(m), Q_(CE) may be detected usingcharge amplifiers 34, such that a voltage output corresponding to then^(th) of N second sensing electrodes x_(n), 20 is Vx_(n), and so forth.Similar to the first method, if the charge amplifier 34 gains areapproximately equal to a common gain value G, i.e.Gx_(n)≈Gy_(m)≈G_(CE)≈G, then Equation (23) may be re-expressed in termsof charge amplifier 34 voltage outputs as:

$\begin{matrix}{{Fx_{n}} = {\frac{Vx_{n}}{G} - {\frac{k_{n}}{C_{CE}}( {\frac{V_{CE}}{G} - F_{CE}} )}}} & (25)\end{matrix}$

Similarly, Equation (24) may be re-expressed in terms of chargeamplifier 34 voltage outputs as:

$\begin{matrix}{{Fy_{m}} = {\frac{{Vy}_{m}}{G} - {\frac{h_{m}}{C_{CE}}( {\frac{V_{CE}}{G} - F_{CE}} )}}} & (26)\end{matrix}$

Experimental Data

Referring also to FIG. 11, experimental data illustrating externallyinduced charges on a sensing electrode x_(n), y_(m) and the commonelectrode 15 are shown.

A first voltage signal 35 (solid line) corresponds to a charge amplifier34 output measured for the common electrode 15. A second voltage signal36 (dashed line) corresponds to a charge amplifier output measured for asensing electrode x_(n), y_(m). The signals 35, 36 shown in FIG. 11 wereobtained using an object 33 in the form of a digit charged to anelectrostatic potential and held nearly touching a touch panel 10. Nopressure was applied to the touch panel 10.

It may be observed that in FIG. 11, the first and second voltage signals35, 36 have corresponding signs (in other words the signals havesubstantially the same polarities at a given time).

Referring also to FIG. 12, experimental data illustrating piezoelectricinduced charges on a sensing electrode x_(n), y_(m) and the commonelectrode are shown.

The first and second voltage signals 35, 36 correspond respectively tothe common electrode 15 and a sensing electrode x_(n), y_(m) in the sameway as FIG. 11. However, the data shown in FIG. 12 was captured inresponse to tapping the touch panel 10 using a non-conductive object, inorder to generate piezoelectric pressure signals which are substantiallyfree of noise from external electric fields.

It may be observed that in FIG. 12, the first and second voltage signals35, 36 have opposite signs (in other words the signals havesubstantially opposite polarities at a given time).

The observed polarities do not precisely correspond to the ideal case ineither of FIG. 11 or 12, which is thought to be as a result of smallvariations in DC offsets and other sources of measurement error.

Second Apparatus

Apparatuses for combined capacitance and pressure sensing have beendescribed in WO 2016/102975 A2, in particular with reference to FIGS. 22to 26 of this document.

In order to aid understanding of the second apparatus 37 (FIG. 13) ofthe present specification, it may be helpful to briefly discuss theoperation of apparatuses for combined capacitance and pressure sensingas described in WO 2016/102975 A2. The discussion hereinafter is madewith reference to the structure of the first touch panel 10 of thepresent specification.

The layer of piezoelectric material 16 is poled. Consequently, thepressure applied by a user interaction will cause a strain which inducesa polarisation P of the layer of piezoelectric material 16. Thepolarisation P of the layer of piezoelectric material 16 results in aninduced electric field E_(n) , which has a component E_(z) in thethickness direction. The deformation which produces the polarisation Pmay result from a compression or a tension. The deformation whichproduces the polarisation P may be primarily an in-plane stretching ofthe piezoelectric material layer 16 in response to the applied pressureof a user interaction.

The induced electric field E_(p) produces a potential difference betweenthe common electrode 15 and any one of the sensing electrodes 14, 20.Electrons flow on or off the electrodes 14, 15, 20 until the inducedelectric field E_(p) is cancelled by an electric field E_(q) produced bythe charging of the electrodes 14, 15, 20. In other words, the electricfield E_(q) results from the charges Fx_(n), Fy_(m), F_(CE).

When the touch panel 10 is used for combined capacitance and pressuresensing, signals received from the sensing electrodes 14, 20 generallytake the form of a superposition of a piezoelectric signal pressuresignal and an applied or sensed capacitance measurement signal.Apparatuses for combined capacitance and pressure sensing as describedin WO 2016/102975 A2, in particular with reference to FIGS. 22 to 26,operate by using first and second frequency dependent filters (notshown) to separate signals received from the sensing electrodes 14, 20into a first component including capacitance information and a secondcomponent including piezoelectric pressure information. The first andsecond frequency dependent filters (not shown) may be physical filters,or may be applied during digital signal processing. This is possiblebecause piezoelectric pressure signals and capacitance measurementsignals generally have different, separable frequency contents.

For example, mutual capacitances between a pair of sensing electrodes14, 20 may typically fall within the range of 0.1 to 3000 pF or more,and preferably 100 to 2500 pF. In order to effectively couple tocapacitances in this range, a capacitance measurement signal maytypically have a base frequency of greater than or equal to 10 kHz,greater than or equal to 20 kHz, greater than or equal to 50 kHz orgreater than or equal to 100 kHz. By contrast, piezoelectric pressuresignals typically include a broadband frequency content spanning a rangefrom several Hz to several hundreds or thousands of Hz. This is at leastin part because piezoelectric pressure signals arise from userinteractions by a human user.

Referring also to FIG. 13, a second apparatus 37, for combinedcapacitance and differential piezoelectric pressure measurements, isshown.

In the apparatuses described in WO 2016/102975 A2, the first and secondfrequency dependent filters (not shown) are implemented in hardware as apart of front end modules, or in the digital domain, for example by acontroller.

By contrast, the second apparatus 37 of the present specificationimplements first frequency dependent filters to select the firstpiezoelectric pressure signals 29 using analog-to-digital converters(ADC) 38 a, 38 b which are synchronised with a capacitance measurementsignal 39 at a first sampling frequency f_(piezo). The second apparatus37 implements a second frequency dependent filter in the digital domainto obtain capacitance signals 27. For example, by application of adigital high-pass filter, or by using the more recently sample value, orvalues, of the first piezoelectric pressure signals 29 to provide abaseline.

The second apparatus 37 includes a first touch panel 10 and a touchcontroller 40 for combined capacitance and differential pressuresensing. The second apparatus 37 may be incorporated into an electronicdevice (not shown) such as, for example, a mobile telephone, a tabletcomputer, a laptop computer and so forth. The first touch panel 10 maybe bonded overlying the display (not shown) of an electronic device (notshown). In this case, the materials of the first touch panel 10 shouldbe substantially transparent. A cover lens (not shown) may be bondedoverlying the first touch panel 10. The cover lens (not shown) ispreferably glass but may be any transparent material.

The touch controller 40 includes a controller 25. The touch controller40 also includes a first circuit 23 including a pair of amplifiermodules 41 a, 41 b a pair of multiplexers 42 a, 42 b, a pair of primaryADCs 38 a, 38 b and a pair of secondary ADCs 43 a, 43 b. The touchcontroller also includes a second circuit 24 including a commonelectrode charge amplifier 44 and a common electrode ADC 45. Thecontroller 25 may communicate with one or more processors (not shown) ofan electronic device (not shown) using a link 46. The controller 25includes a signal source (not shown) for providing a driving capacitancemeasurement signal 39, V_(sig)(t) (FIG. 14, also referred to as “drivingsignal” for brevity hereinafter) to one or both of the amplifier modules41 a, 41 b).

The second apparatus 37 will be described with reference to an examplein which the driving signal 39, V_(sig)(t) is supplied to the firstamplifier module 41 a, such that the first sensing electrodes 14 aretransmitting, Tx electrodes, and the second sensing electrodes 20 arereceiving, Rx electrodes.

Each amplifier module 41 a, 41 b includes a number of separate chargeamplifiers 34. Each charge amplifier 34 of the first amplifier module 41a is connected to a corresponding first sensing electrode 14 via aconductive trace 26. The output of each charge amplifier 34 of the firstamplifier module 41 a is connected to a corresponding input of the firstmultiplexer 42 a. In this way, the first multiplexer 42 a may output anamplified signal 47 a corresponding to an addressed first sensingelectrode 14.

The first primary ADC 38 a receives the amplified signal 47 acorresponding to a presently addressed first sensing electrode 14 fromthe first multiplexer 42 a output. The amplified signal 47 acorresponding to a presently addressed first sensing electrode 14includes a superposition of the driving signal 39, V_(sig)(t) and apiezoelectric pressure signal 29, V_(piezo)(t). The first primary ADC 38a also receives a first synchronisation signal 48 a from the controller25 (also referred to as a “clock signal”). The first synchronisationsignal 48 a triggers the first primary ADC 38 a to obtain samples at afirst sampling frequency f_(piezo) and at times corresponding to theamplitude of the driving signal 39, V_(sig)(t) being substantially equalto a ground, common mode or minimum value. In this way, the firstprimary ADC 38 a may obtain a first filtered signal 49 a in the form ofa sampled signal which corresponds approximately to a piezoelectricpressure signal 29, V^(piezo)(t) generated by the first sensingelectrode 14 connected with the first multiplexer 42 a. The firstsynchronisation signal 48 a need not trigger the first primary ADC 38 ato obtain samples during every single period of the driving signal 39,V_(sig)(t), and instead may trigger the first primary ADC 38 a to obtainsamples during, for example, every other period, every tenth period,every hundredth period and so forth.

For example, referring also to FIGS. 14A to 14C, an example of obtaininga piezoelectric pressure signal 29 in the form of the first filteredsignal 49 a is illustrated.

For visual purposes, in FIGS. 14A to 14C, the driving signal 39,V_(sig)(t) and a superposed piezoelectric pressure signal 29,V_(piezo)(t) have been illustrated with much smaller disparities infrequency and amplitude than would be expected in practice. In practice,the driving signal 39, V_(sig)(t) would be expected to have asignificantly larger amplitude and to vary at a frequency several ordersof magnitude larger than the piezoelectric pressure signal 29,V_(piezo)(t).

Referring in particular to FIG. 14A, an example of a driving signal 39,V_(sig)(t) of base frequency f_(d) may take the form a pulsed wave witha 50:50 duty ratio and a period of 1/f_(d). In this example, the firstsynchronisation signal 48 a triggers the first primary ADC 38 a atapproximately the midpoint of the driving signal 39, V_(sig)(t) minimum,or zero, period. For example, the first primary ADC 38 a may obtain asample at times t₁, t₂=t₁+1/f_(d), t₃=t₁+2/f_(d) and so forth.

Referring in particular to FIG. 14B, with the first sensing electrodes14 acting as transmitter electrodes, Tx, and the second sensingelectrodes 20 acting as receiving electrodes, Rx, the amplified signal47 a may be approximated as a superposition of a piezoelectric pressuresignal 29, V_(piezo)(t) and the driving signal 39 V_(sig)(t). The firstsynchronisation signal 48 a triggers sampling of the amplified signal 47a at times when the contribution of the driving signal 39, V_(sig)(t) tothe amplified signal 47 a is substantially equal to a ground, commonmode or minimum value. In this way, a sampling of substantially only thepiezoelectric pressure signal 29, V_(piezo)(t) may be obtained.

Referring in particular to FIG. 14C, the first filtered signal 49 a thentakes the form of a sequence of samplings of the piezoelectric pressuresignal 20 V_(piezo)(t) at times t₁, t₂, t₃ and so forth.

The first secondary ADC 43 a receives the amplified signal 47 acorresponding to a presently addressed first sensing electrode 14 fromthe first multiplexer 42 a output. The first secondary ADC 43 a samplesthe amplified signal 47 a at a sampling frequency f_(cap), which is atleast several times the base frequency f_(d) of the driving signal 39,V_(sig)(t). The first secondary ADC 43 a outputs a digitised amplifiedsignal 50 a to the controller 25. The controller 25 receives thedigitised amplified signal 50 a and applies a digital high pass filterto obtain a second filtered signal in the digital domain. The secondfiltered signal corresponds to capacitance signals 27.

Alternatively, since the piezoelectric pressure signal 29, V_(piezo)(t)typically varies at frequencies several orders of magnitude lower thanthe base frequency f_(d) of the driving signal 39, V_(sig)(t), thecontroller 25 may treat the most recently sampled value of the firstfiltered signal 49 a, for example V_(piezo)(t₃), as an additional offsetand subtract this value from the digitised amplified signal 50 a. Moreaccurate baseline corrections may be employed, for example, linearinterpolation based on the two most recent sampled values of the firstfiltered signal 49 a, or quadratic interpolation based on the three mostrecently sampled values of the first filtered signal 49 a.

The primary and secondary ADCs 38 a, 43 a may be the same. However, itmay be advantageous for the primary and secondary ADCs 38 a, 43 a to bedifferent. In particular, the primary ADC 38 a may be optimised for thedynamic range of the piezoelectric pressure signals 29, V_(piezo)(t),without the need to measure the larger amplitudes corresponding to thedriving signal V_(sig)(t). Furthermore, because the first samplingfrequency f_(piezo) should be at most equal to the base frequency f_(d)of the capacitance measurement signal 39, V_(sig)(t), a lower bandwidthis required for the primary ADC 38 a compared to the secondary ADC 43 a.For cost sensitive applications, this enables use of cheaper, ADCs forthe primary ADC 38 a. By contrast, for performance applications, thisenables the use of more precise ADCs capable of differentiating a largernumber of signal levels within the same dynamic range (a 16-bit ADC istypically slower than an 8-bit ADC all else being equal).

The processing of signals from the second sensing electrodes 20 issimilar to that of signals from the first sensing electrodes 14, exceptthat because the second sensing electrodes 20 are the receiving, Rxelectrodes, a second synchronisation signal 48 b for the second primaryADC 38 b may be offset with respect to the first synchronisation signal48 a.

Each charge amplifier 34 of the second amplifier module 41 b isconnected to a corresponding second sensing electrode 20 via aconductive trace 26, and the output of each charge amplifier 34 of thesecond amplifier module 41 b is connected to a corresponding input ofthe second multiplexer 42 b). In this way, the second multiplexer 42 bmay output an amplified signal 47 b corresponding to an addressed secondsensing electrode 20.

The amplified signal 47 b corresponding to a presently addressed secondsensing electrode 20 includes a superposition of a received capacitancemeasurement signal (not shown) V_(meas)(t) and a piezoelectric pressuresignal 29, V_(piezo)(t). The received capacitance measurement signalV_(meas)(t) (referred to as a “received signal” for brevity hereinafter)is the driving signal 39, V_(sig)(t) as coupled to the addressed secondsensing electrode 20 by a mutual capacitance between the addressedsecond sensing electrode 20 and a first sensing electrode 14. Thereceived signal V_(meas)(t) is related to and has a similar form to thedriving signal 39, V_(sig)(t), and in particular has substantially thesame frequency contents. However, the received signal V_(meas)(t) mayinclude a change in amplitude and/or a change in phase compared to thedriving signal V_(sig)(t). The second primary ADC 38 b receives a secondsynchronisation signal 48 b from the controller 25 (also referred to asa “clock signal”). The second synchronisation signal 48 b triggers thesecond primary ADC 38 b to obtain samples at the sampling frequencyf_(piezo) and at times corresponding to the amplitude of the receivedsignal V_(meas)(t) being substantially equal to a ground, common mode orminimum value. Depending on the form of the driving signal 39,V_(sig)(t) and the typical phase shifts between driving signals 39,V_(sig)(t) and the received signals V_(meas)(t), there are severalpossible relationships between the first and second synchronisationsignals 48 a, 48 b.

When the received signal V_(meas)(t) is approximately in phase with thedriving signal 39, V_(sig)(t), the second synchronisation signal 48 bmay be the same as the first synchronisation signal 48 a. The secondsynchronisation signal 48 b will trigger sampling of the amplifiedsignal 47 b at times when the contribution of the received signalV_(meas)(t) to the amplified signal 47 b is substantially equal to aground, common mode or minimum value. In this way, a sampling ofsubstantially only the piezoelectric pressure signal 29, V_(piezo)(t)may be obtained.

Similarly, for a driving signal 39, V_(sig)(t) in the form of a pulsedwave as shown in FIG. 14A, small phase shifts φ of up to about φ±π/2between the received signal V_(meas)(t) and the driving signal 39,V_(sig)(t) may be accommodated without requiring any offset between thefirst and second synchronisation signals 48 a, 48 b. For a pulsed wave,such phase shifts can be tolerated because the driving signal 39,V_(sig)(t) and received signal V_(meas)(t) are each substantially equalto zero for half of each period.

For larger phase shifts φ or different, non-square, waveforms of thedriving signal 39, V_(sig)(t), the second synchronisation signal 48 bmay be offset with respect to the first synchronisation signal 48 a suchthat, within the range of capacitances expected/measured for thecorresponding touch panel 10, the second synchronisation signal 48 btriggers the second primary ADC 38 b during a period of low or zerosignal level of the received signal V_(meas)(t). In other words, thesecond synchronisation signal 48 b may synchronise the sampling of thesecond primary ADC 38 b to the received signal V_(meas)(t), instead ofthe driving signal 39, V_(sig)(t).

Alternatively, the second synchronisation signal 48 b could be generatedin response to a condition on the received signal V_(meas)(t). Forexample, a simple comparator circuit could be used to generated thesecond synchronisation signal 48 b in response to the received signalV_(meas)(t) dropping to within a pre-calibrated range of ground, commonmode or a minimum value. A circuit triggering the second synchronisationsignal 48 b may include a delay timer.

In this way, the second primary ADC 38 b may obtain a second filteredsignal 49 b in the form of a sampled signal which correspondsapproximately to a piezoelectric pressure signal 29, V_(piezo)(t)generated by the second sensing electrode 20 connected via the secondmultiplexer 42 b. The second synchronisation signal 48 b need nottrigger the second primary ADC 38 b to obtain samples during everysingle period of the driving signal 39, V_(sig)(t) or measured signalV_(meas)(t), and instead may trigger the second primary ADC 38 b toobtain samples during, for example, every other period, every tenthperiod, every hundredth period and so forth.

The controller 25 may also provide a second synchronisation signal 51 tothe multiplexers 42 a, 42 b and/or amplifiers 34. The secondsynchronisation signal 51 may cause the multiplexers 42 a, 42 b toaddress each combination of first and second sensing electrodes 14, 20according to a sequence determined by the controller 25. In this way,the touch controller 25 may receive amplified signals 47 a, 47 b fromeach pairing of first and second sensing electrodes 14, 20 according toa sequence determined by the controller 25. The sequence may bepre-defined, for example, the sequence may select each pair of a firstsensing electrode 14 and a second sensing electrode 20 once beforerepeating. The sequence may be dynamically determined, for example, whenone or more user interactions are detected, the controller 25 may scanthe subset of first and second sensing electrodes 14, 20 adjacent toeach detected user interaction in order to provide faster and/or moreaccurate tracking of user touches.

The common electrode charge amplifier 44 receives signals from thecommon electrode 15 and generates a common electrode amplified signal52. The common electrode ADC 45 receives the common electrode amplifiedsignal 52 and samples it at the piezoelectric sampling frequencyf_(piezo) to generate the second piezoelectric signal 30. Optionally,the common electrode ADC 45 is also synchronised by a thirdsynchronisation signal 48 c, which may be identical to or offset fromthe first synchronisation signal, so as to sample the secondpiezoelectric signal 30 at times corresponding to ground, common mode ora minimum value of the driving signal 39, V_(sig)(t) and/or ground,common mode or a minimum value the received signal V_(meas)(t).Synchronisation of the common electrode ADC 45 may help to reduce oravoid cross-talk from the capacitance measurements.

Based on the obtained filtered signals 49 a, 49 b the controller 25 maycalculate pressure values 32 a, 32 b corresponding to the addressedfirst and second sensing electrodes 14, 20. The pressure values 32 a, 32b are determined based on the first and second piezoelectric pressuresignals 29, 30, using the first and/or second methods describedhereinbefore. The pressure values 32 a, 32 b may be output via the link46.

As mentioned hereinbefore, the controller 25 provides the driving signal39, V_(sig)(t) to each amplifier 34 of the first amplifier module 41 a.An input of each amplifier 34 of the first amplification module 41 a maybe used to drive the corresponding first sensing electrode 14 of thefirst touch panel 10 using the driving signal 39, V_(sig)(t). Based onthe driving signal 39, V_(sig)(t) and the first and second digitisedamplified signals 50 a, 50 b obtained by the controller 25, thecontroller 25 calculates capacitance values 27 and/or touch data 31based on the mutual-capacitance between the addressed first and secondsensing electrodes 14, 20. The capacitance values 27 and/or touch data31 may be output via the link 46.

Referring also to FIG. 15, an example of one configuration of chargeamplifiers 34 a, 34 b, 44 suitable for use in the second apparatus isshown.

In one configuration, each charge amplifier 34 a, 34 b, 44 includes anoperational amplifier OP having an inverting input, a non-invertinginput and an output.

For example, each charge amplifier 34 a forming part of the firstamplifier module 41 a includes an operational amplifier OP having aninverting input for coupling to a corresponding first sensing electrode14 via an input resistance R_(i) and a first switch SW1 connected inseries. The non-inverting input of the operational amplifier OP isconnected to the driving signal 39, V_(sig)(t). The driving signal 39,V_(sig)(t) may be provided by the controller 25, by a separate module(not shown) of the second apparatus 37 or may be received into thesecond apparatus 37 from an external source. Since the inverting inputwill be at practically the same voltage as the non-inverting input, theinverting input can be caused to drive the corresponding first sensingelectrode 14. A feedback network of the charge amplifier 34 a includes afeedback resistance R_(f), a feedback capacitance C_(f) and a secondswitch SW2 connected in parallel between the inverting input and theoutput of the operational amplifier OP. The output of the operationalamplifier OP provides the amplified signal 47 a.

Each charge amplifier 34 b forming part of the second amplifier module41 b is the same as each charge amplifier 34 a of the first amplifiermodule 41 a, except that the non-inverting input of the operationalamplifier OP is coupled to a common mode voltage V_(CM) instead of thedriving signal 39, V_(sig)(t), and in that the inverting input isconnected to a second sensing electrode 20 instead of a first sensingelectrode 14.

The common electrode charge amplifier 44 is the same as the chargeamplifiers 34 b forming part of the second amplifier module 41 b, exceptthat the inverting input of the common electrode charge amplifier 44 isconnected to the common electrode 15 and the common electrode chargeamplifier 44 omits the first switch SW1.

Other terminals of the operational amplifiers OP, such as power supplyterminals, may be present, but are not shown in this or other schematiccircuit diagrams described herein.

The second switches SW2 permit the corresponding feedback capacitorsC_(f) to be discharged. The opening and closing of the second switchesSW2 may be governed by the second synchronisation signal 51 provided bythe controller 25. In this way, the feedback capacitors C_(f) of eachcharge amplifier 34 a, 34 b may be periodically discharged in order toreset the feedback network of the operational amplifier OP to preventexcessive drift. Optionally, the second switch SW2 of the commonelectrode charge amplifier 44 may also be synchronised using the secondsynchronisation signal 51.

The first switches SW1 may be controlled by the second synchronisationsignal 51 provided by the controller 25 to enable an amplifier 34 a, 34b to be connected or disconnected from the corresponding sensingelectrode 14, 20 if required.

The first sensing electrodes 14 need not be transmitting, Tx electrodesand the second sensing electrodes 20 receiving, Rx electrodes.Alternatively, the controller 25 may provide the driving signal 39,V_(sig)(t) to the second amplifier module 41 b so that the secondsensing electrodes 20 are transmitting, Tx electrodes and the receivedsignals V_(meas)(t) are detected using the first sensing electrodes 14.

In other examples, the second apparatus 37 need not be configured formutual capacitance measurements, and may instead be configured tomeasure self-capacitances of each first and second sensing electrode 14,20. In this case, a self-capacitance measurement signal (not shown) maybe provided to both the first and second amplifier modules 41 a, 41 b.

Second Touch Panel

In the first touch panel 10, the first and second sensing electrodes 14,20 have been shown in the form of elongated rectangular electrodes.However, other shapes may be used.

Referring also to FIG. 16, a second touch panel 53 having an alternativegeometry of the first and second sensing electrodes 14, 20 is shown.

Instead of being rectangular, each first sensing electrode 14 mayinclude several pad segments 54 evenly spaced in the first direction xand connected to one another in the first direction x by relativelynarrow bridging segments 55. Similarly each second sensing electrode 20may comprise several pad segments 56 evenly spaced in the seconddirection y and connected to one another in the second direction y byrelatively narrow bridging segments 57. The pad segments 54 of the firstsensing electrodes 14 are diamonds having a first width W₁ in the seconddirection y and the bridging segments 55 of the first sensing electrodes14 have a second width W2 in the second direction y. The pad segments 56and bridging segments 57 of the second sensing electrodes 20 have thesame respective shapes and widths W1, W2 as the first sensing electrodes14.

The first and second sensing electrodes 14, 20 are arranged such thatthe bridging segments 57 of the second sensing electrodes 20 overlie thebridging segments 55 of the first sensing electrodes 14. Alternatively,the first and second sensing electrodes 14, 20 may be arranged such thatthe pad segments 56 of the second sensing electrodes 20 overlie the padsegments 54 of the first sensing electrodes 14. The pad segments 54, 56need not be diamond shaped, and may instead be circular. The padsegments 54, 56 may be a regular polygon such as a triangle, square,pentagon or hexagon. The pad segments 54, 56 may be I shaped or Zshaped.

The alternative geometry of the second touch panel 53 is equallyapplicable in combination with the first or second apparatus 22, 37.

Third Touch Panel

Referring also FIG. 17, a third touch panel 58 may be used incombination with the first or second apparatus 22, 37.

The third touch panel 58 is substantially the same as the first touchpanel 10 except that the third touch panel 58 does not include thesecond layer structure 17 and the second sensing electrodes 20 aredisposed on the first face 12 of the first layer structure 11 inaddition to the first sensing electrodes 14. Each first sensingelectrode 14 is a continuous conductive region extending in the firstdirection x. For example, each first sensing electrode 14 may includeseveral pad segments 59 evenly spaced in the first direction x andconnected to one another in the first direction x by relatively narrowbridging segments 60. Each second sensing electrode 20 may compriseseveral pad segments 61 evenly spaced in the second direction y.However, the pad segments 61 of the second sensing electrodes 20 aredisposed on the first face 12 of the first layer structure 11 and areinterspersed with, and separated by, the first sensing electrodes 14.The pad segments 61 corresponding to each second sensing electrode 20are connected together by conductive jumpers 62. The jumpers 62 eachspan a part of a first sensing electrode 14 and the jumpers 62 areinsulated from the first sensing electrodes 14 by a thin layer ofdielectric material (not shown) which may be localised to the areaaround the intersection of the jumper 62 and the first sensing electrode14.

Alternatively, a thin dielectric layer (not shown) may overlie the firstface 12 of the first layer structure 11, the first sensing electrodes 14and the conductive pads 61 of the second sensing electrodes 20.Conductive traces (not shown) extending in the second direction y may bedisposed over the dielectric layer (not shown), each conductive trace(not shown) overlying the pad segments 61 making up one second sensingelectrode 20. The overlying conductive traces (not shown) may connectthe pad segments 61 making up each second sensing electrode 20 usingvias (not shown) formed through the thin dielectric layer (not shown).

Modifications

It will be appreciated that many modifications may be made to theembodiments hereinbefore described. Such modifications may involveequivalent and other features which are already known in the design,manufacture and use of pressure and/or projected capacitance sensingtouch panels and which may be used instead of or in addition to featuresalready described herein. Features of one embodiment may be replaced orsupplemented by features of another embodiment.

Third Apparatus

Referring also to FIG. 18, a third apparatus 66 includes the first touchpanel 10 and a second controller 67 for combined pressure andcapacitance sensing.

The second controller 67 is the same as the first controller 40, exceptthat in the second controller 67 the input signals from a first sensingelectrode 14 are connected to a single charge amplifier 34 a by a firstmultiplexer 42 a. The charge amplifier 34 a outputs the first amplifiedsignal 47 a, which is processed by the first primary ADC 38 a, the firstsecondary ADC 43 a and the controller 25 in the same way as for thefirst controller 40. Similarly, the input signals from a second sensingelectrode 20 are connected to a single charge amplifier 34 b by a secondmultiplexer 42 b. The charge amplifier 34 b outputs the second amplifiedsignal 47 b, which is processed by the second primary ADC 38 b, thesecond secondary ADC 43 b and the controller 25 in the same way as forthe first controller 40. The acquisition and processing of signals fromthe common electrode 15 is the same as for the second apparatus 37.

In the same way as the first controller 40, use of primary and secondaryADCs 38, 43 is not essential. Instead, a single ADC (not shown) which iscapable of alternating operation at the piezoelectric and capacitivesampling frequencies f_(piezo), f_(cap), so as to obtain the signals 49,50 sequentially.

Fourth Apparatus

Referring also to FIG. 19, a fourth apparatus 68 for combined pressureand capacitance sensing is shown.

The fourth apparatus 68 includes the first touch panel 10 and ameasurement circuit including a first circuit 23 in the form of acapacitive touch controller 69, an impedance network 70 and a chargeamplifier 34, a second circuit 24 in the form of a common electrodecharge amplifier 44, and a controller 25.

The capacitive touch controller 69 may be a standard, commerciallyavailable device configured for either self-capacitance measurements ofthe individual first and second sensing electrodes 14, 20, for mutualcapacitance measurements between pairings of first and second sensingelectrodes 14, 20, or both. Suitable devices for providing thecapacitive touch controller include, without being limited to, a SolomonSystech maXTouch (RTM) controller, a Cypress Semiconductor CapSense(RTM) controller, a Synaptics ClearPad (RTM) controller, or otherdevices with comparable functionality. Each measurement terminal of thecapacitive touch controller 69 is coupled to a corresponding first orsecond sensing electrode 14, 20 via an input capacitance C_(in) and aconductive trace 26. The input capacitances C_(in) may typically have avalue ranging between about 100 pF and about 1 nF. The capacitive touchcontroller 69 may operate in any standard way to obtain capacitancevalues 27 which are passed to the controller 25.

Instead of measuring a first signal 29 corresponding to each sensingelectrode 14, 20 or groups of adjacent sensing electrodes 14, 20, in thefourth apparatus 68 the impedance network 70 couples all of the sensingelectrodes 14, 20 to an input of a single charge amplifier 34. In otherwords, in the fourth apparatus 68, the summation to obtain Q_(sen)according to Equation (11) (or V_(sen) according to Equation (17)) ismoved from the digital domain into the analogue domain. Each sensingelectrode 14, 20 is coupled to the input of the charge amplifier 34 byan input resistance R_(in). The input resistances R_(in) aresufficiently large to suppress cross-talk between measurement channelsof the capacitive touch controller 69. Input resistances R_(in) maytypically have a value ranging between about 10 kΩ and about 100 kΩ.

The controller 25 receives the first piezoelectric signal 29 and thesecond piezoelectric signal 30. The controller 25 may estimate thecorrected piezoelectric signal 32 based on the total piezoelectriccharge F_(CE) using, for example, Equation (13) or Equation (21). Thetotal piezoelectric charge F_(CE) provides an estimate of the totalapplied pressure. The controller 25 uses the capacitance values 27 todetermine touch location data 31. Alternatively, if the capacitive touchcontroller 69 provides suitable functionality, the capacitive touchcontroller 69 may determine touch location data 31 internally and outputthe touch location data 31 to the controller 25. The controller 25 mayuse the touch location data 31 from the capacitive touch controller 69to look-up position dependent coefficients for the weighted differencecalculation of the total piezoelectric charge F_(CE).

In this way, the fourth apparatus 68 may be used to provide an estimateof total applied pressure concurrently with standard capacitancemeasurements obtained using a conventional capacitive touch controller69. This may allow a conventional capacitive touch system to beaugmented with an estimate of global applied force with the addition ofthe piezoelectric layer 16, the common electrode 15, the impedancenetwork 70, and a pair of charge amplifiers 34, 44.

In a modification of the fourth apparatus 68, the input capacitancesC_(in) may be replaced with an array of switches (not shown) which maybe used to disconnect the sensing electrodes 14, 20 from the capacitivetouch controller 69. In such a modification, the measurements ofcapacitance values 27 and piezoelectric signals 29, 30 are notconcurrent and are instead time multiplexed. For example, in a first,capacitance measurement period the sensing electrodes 14, 20 may beconnected to the capacitive touch controller 69 to obtain capacitancevalues 27, whilst in a second, pressure measurement period the sensingelectrodes 14, 20 may be disconnected from the capacitive touchcontroller 69 whilst the first and second piezoelectric signals 29, 30are obtained. During the capacitance measurement period, switches may beused to short feedback networks of the charge amplifiers 34, 44 (similarto SW2 in FIG. 15). The capacitance measurement period and the pressuremeasurement period repeat in a loop.

Although illustrated with reference to the first touch panel 10, thefourth apparatus 68 may alternatively use the second touch panel 53, thethird touch panel 58, or any other suitable touch panel which includes apiezoelectric layer 16 arranged between one or more common electrodes 15and a number of sensing electrodes 14, 20.

Fifth Apparatus

Referring also to FIG. 20, a fifth apparatus 71 for combined pressureand capacitance sensing is shown.

The fifth apparatus 71 is the same as the fourth apparatus 68, exceptthat the measurement circuit additionally includes a differentialvoltage amplifier 72 which receives the first and second piezoelectricsignals 29, 30 as inputs. In this way, the corrected piezoelectricsignal 32 is obtained directly in the analogue domain. The weighting ofthe first and second piezoelectric signals 29, 30 may be accomplished,for example, by varying input resistances and/or feedback networks ofthe differential voltage amplifier 72. Compared to the fourth apparatus,the fifth apparatus 71 may relatively reduce the computational load onthe controller 25. However, the fifth apparatus is unable to useposition dependent coefficients for the weighted difference, for examplebased on the touch location data 31.

In a modification of the fifth apparatus 68, the input capacitancesC_(in) may be replaced with an array of switches (not shown) which maybe used to disconnect the sensing, electrodes 14, 20 from the capacitivetouch controller 69. In such a modification, the measurements ofcapacitance values 27 and corrected piezoelectric signal 32 are notconcurrent and are instead time multiplexed. For example, in a first,capacitance measurement period the sensing electrodes 14, 20 may beconnected to the capacitive touch controller 69 to obtain capacitancevalues 27. In a second, pressure measurement period the sensingelectrodes 14, 20 may be disconnected from the capacitive touchcontroller 69 and the corrected piezoelectric signal 32 may be obtainedfrom the differential voltage amplifier 72. During the capacitancemeasurement period, switches may be used to short feedback networks ofthe charge amplifiers 34, 44 (similar to SW2 in FIG. 15).

Although illustrated with reference to the first touch panel 10, thefifth apparatus 71 may alternatively use the second touch panel 53, thethird touch panel 58, or any other suitable touch panel which includes apiezoelectric layer 16 arranged between one or more common electrodes 15and a number of sensing electrodes 14, 20.

Although the examples described hereinbefore have been primarilydescribed with reference to combined piezoelectric and capacitive touchpanels and apparatus, it should be appreciated that the first and secondmethods of the specification may also be used in a touch panel apparatuswhich does not measure capacitances and which only measurespiezoelectric pressure signals. Provided that a piezoelectric touchpanel allows determining the coordinates x, y of a user interaction,appropriate pre-calibrated values of the coefficients k_(n)(x,y),h_(m)(x,y), and/or C_(CE)(x,y) may be retrieved and applied in Equations(13), (15), (16), or the voltage equivalents thereof.

Although in the examples described hereinbefore a first piezoelectricpressure signal 29 is generated corresponding to each first or secondsensing electrode 14, 20, this need not be the case. In other examples,signals from a group of two or more adjacent sensing electrodes 14, 20may be combined by a single charge amplifier 34, to produce a firstpiezoelectric pressure signal 29 corresponding to the charge induced onthe group of two or more adjacent sensing electrodes 14, 20.

Although the examples described hereinbefore have been primarilydescribed with reference to touch panels in which first and secondsensing electrodes 14, 20 are elongated in perpendicular directions,this need not be the case. In other examples, the second sensingelectrodes 20 may be omitted and the first sensing electrodes 14 maytake the form of a two dimensional array of discrete touch pads. Infurther examples, the second sensing electrodes 20 may be omitted andthe first sensing electrodes 14 may take the form of discrete touchpanels arranged in irregular arrays, disposed at arbitrary locations, ora arranged with a mixture of some first sensing electrodes 14 arrangedin one or more regular arrays and other first sensing electrodes 14disposed in one or more irregular arrays or at arbitrary locations. Thefirst and second methods of the present specification may still be usedwith such examples.

Although claims have been formulated in this application to particularcombinations of features, it should be understood that the scope of thedisclosure of the present invention also includes any novel features orany novel combination of features disclosed herein either explicitly orimplicitly or any generalization thereof, whether or not it relates tothe same invention as presently claimed in any claim and whether or notit mitigates any or all of the same technical problems as does thepresent invention. The applicant hereby gives notice that new claims maybe formulated to such features and/or combinations of such featuresduring the prosecution of the present application or of any furtherapplication derived therefrom.

1. An apparatus for processing signals from a touch panel, the touchpanel comprising a layer of piezoelectric material disposed between aplurality of sensing electrodes and at least one common electrode, theapparatus comprising: a first circuit for connection to the plurality ofsensing electrodes and configured to generate one or more first pressuresignals, wherein each first pressure signal corresponds to one or moresensing electrodes and is indicative of a pressure acting on the touchpanel proximate to the corresponding one or more sensing electrodes; asecond circuit for connection to the at least one common electrode andconfigured to generate a second pressure signal indicative of a totalpressure applied to the touch panel; a controller configured todetermine an estimate of the total pressure based on a weighteddifference of the second pressure signal and a sum over the one or morefirst pressure signals.
 2. The apparatus according to claim 1, whereinthe controller is further configured to determine a location at whichpressure is applied to the touch panel; wherein a coefficient used forthe weighted difference of the second pressure signal and the sum overthe one or more first pressure signals depends upon the location.
 3. Theapparatus according to claim 1, wherein the controller determines theestimate of the total pressure using the equation:F _(CE)=(1−C _(CE))Q _(CE) −C _(CE) Q _(sen) in which F_(CE) is apiezoelectric charge induced on the at least one common electrode,Q_(CE) is a charge measured on the at least one common electrode,Q_(sen) is the sum of charges measured on all of the plurality ofsensing electrodes, and C_(CE) is a pre-calibrated constant having avalue between zero and unity; wherein the estimate of the total pressureis based on F_(CE).
 4. The apparatus according to claim 1, wherein thecontroller is further configured to determine, for each of at least onefirst pressure signal, an estimate of the pressure acting on the touchpanel proximate to the corresponding one or more sensing electrodes,based on the first pressure signal, the second pressure signal and thetotal pressure.
 5. The apparatus according to claim 4, wherein thecontroller is configured to determine a location at which pressure isapplied to the touch panel; wherein one or more coefficients used todetermine the estimate of the pressure acting on the touch panelproximate to the one or more sensing electrodes depends upon thelocation.
 6. The apparatus according to claim 4, wherein the touch panelcomprises a number N of sensing electrodes, and wherein the controllerdetermines the estimate of the pressure acting on the touch panelproximate to the one or more sensing electrodes using the equation:$F_{n} = {Q_{n} - {\frac{k_{n}}{C_{CE}}( {Q_{CE} - F_{CE}} )}}$in which F_(n) is a piezoelectric charge induced on the n^(th) of Nsensing electrodes, F_(CE) is a piezoelectric charge induced on the atleast one common electrode, Q_(n) is a charge measured on the n^(th) ofN sensing electrodes, Q_(CE) is a charge measured on the at least onecommon electrode, C_(CE) is a pre-calibrated constant having a valuebetween zero and unity, and k_(n) is a pre-calibrated constantcorresponding to the n^(th) of N sensing electrodes and having a valuebetween zero and unity; wherein the estimate of the pressure acting onthe touch panel proximate to the one or more sensing electrodes is basedon one or more corresponding values of F_(n).
 7. The apparatus accordingto claim 1, wherein the first circuit is configured to generate, foreach sensing electrode, a capacitance signal indicative of a capacitanceof the sensing electrode; wherein the controller is configured todetermine a location at which pressure is applied to the touch panelbased on the capacitance signals.
 8. The apparatus according to claim 7,wherein generating the first pressure signals and the capacitancesignals comprises separating single signals received from the sensingelectrodes.
 9. The apparatus according to claim 1, wherein the firstcircuit comprises: a capacitive touch controller for connection to thesensing electrodes; a charge amplifier for connection to each of thesensing electrodes via an impedance network, the charge amplifierconfigured to output a first pressure signal corresponding to all of thesensing electrodes; wherein the second circuit comprises: a commonelectrode charge amplifier for connection to the at least one commonelectrode, the common electrode charge amplifier configured to generatethe second pressure signal.
 10. The apparatus according to claim 9,further comprising a differential amplifier configured to receive thefirst pressure signal and the second pressure signal, and to output aweighted difference of the first pressure signal and the second pressuresignal to the controller; wherein the controller is configured todetermine the estimate of the total pressure based on the weighteddifference received from the differential amplifier.
 11. A touch panelsystem comprising: the apparatus according to claim 1; and a touch panelcomprising a layer of piezoelectric material disposed between aplurality of sensing electrodes and at least one common electrode. 12.An electronic device comprising the touch panel system according toclaim
 11. 13. A method of processing signals from a touch panel, thetouch panel comprising a layer of piezoelectric material disposedbetween a plurality of sensing electrodes and at least one commonelectrode, the method comprising: generating one or more first pressuresignals, each first pressure signal based on signals received from oneor more sensing electrodes, and each first pressure signal indicative ofa pressure acting on the touch panel proximate to the corresponding oneor more sensing electrodes; generating, based on signals received fromthe at least one common electrode, a second pressure signal indicativeof a total pressure applied to the touch panel; determining an estimateof the total pressure based on a weighted difference of the secondpressure signal and a sum over the one or more first pressure signals.14. The method according to claim 13, further comprising determining alocation at which pressure is applied to the touch panel; wherein acoefficient used for the weighted difference of the second pressuresignal and the sum over the one or more first pressure signals dependsupon the location.
 15. The method according to claim 13, whereindetermining the estimate of the total pressure applied to the touchpanel comprises using the equation:F _(CE)=(1−C _(CE))Q _(CE) −C _(CE) Q _(sen) in which F_(CE) is apiezoelectric charge induced on the at least one common electrode,Q_(CE) is a charge measured on the at least one common electrode,Q_(sen) is the sum of charges measured on all of the plurality ofsensing electrodes and C_(CE) is a pre-calibrated constant having avalue between zero and unity; wherein the estimate of the total pressureis based on F_(CE).
 16. A The method according to claim 13, furthercomprising determining, for each of at least one first pressure signal,an estimate of the pressure acting on the touch panel proximate to thecorresponding one or more sensing electrodes, based on the firstpressure signal, the second pressure signal and the total pressure. 17.The method according to claim 16, comprising determining a location atwhich pressure is applied to the touch panel; wherein one or morecoefficients used to determine the estimate of the pressure acting onthe touch panel proximate to the one or more sensing electrodes dependsupon the location.
 18. The method according to claim 16, wherein thetouch panel comprises a number N of sensing electrodes, and whereindetermining the estimate of the pressure acting on the touch panelproximate to the one or more sensing electrodes comprises using theequation:$F_{n} = {Q_{n} - {\frac{k_{n}}{C_{CE}}( {Q_{CE} - F_{CE}} )}}$in which F_(n) is a piezoelectric charge induced on the n^(th) of Nsensing electrodes, F_(CE) is a piezoelectric charge induced on the atleast one common electrode, Q_(n) is a charge measured on the n^(th) ofN sensing electrodes, Q_(CE) is a charge measured on the at least onecommon electrode, C_(CE) is a pre-calibrated constant having a valuebetween zero and unity, and k_(n) is a pre-calibrated constantcorresponding to the n^(th) of N sensing electrodes and having a valuebetween zero and unity; wherein the estimate of the pressure acting onthe touch panel proximate to the one or more sensing electrodes is basedon one or more corresponding values of F_(n).
 19. The method accordingto claim 13, comprising generating, based on signals received from eachsensing electrode, a capacitance signal indicative of a capacitance ofthe sensing electrode; the method further comprising determining alocation at which pressure is applied to the touch panel based on thecapacitance signals.
 20. The method according to claim 19, whereingenerating the first pressure signals and the capacitance signalscomprises separating single signals received from the sensingelectrodes.